The invention is directed to a multiple-span bridge support system constructed of reinforced concrete and/or prestressed concrete and including a primary structure for bridging the span between abutments and intermediate supports and made up of elongated support members extending in the long direction of the bridge and having a width transverse of the long direction which amounts to a fraction of the full useful width of the bridge. A secondary structure forming the roadway slab is supported on the primary structure. The invention is also directed to the method of constructing the primary structure and the secondary structure. In addition to the construction of a large bridge with a single large span, the construction of multiple-span bridges made up of many small spans where the roadway is located only slightly elevated above the ground surface is of increasing importance. Basically, to cut down on construction costs in building such bridges it is necessary not only to have a simple and easily determinable static arrangement and to obtain the optimum utilization of the construction materials being employed, but also to provide economical construction methods. In this regard, step-by-step construction methods have been developed in which the construction processes take place successively in multiple sequences.
For reinforced concrete and/or prestressed concrete bridges with large spans, a closed box-type cross-section is preferred for the superstructure. Moreover, in the cross-sections governing the dimensions of the upper roadway slab as well as of the lower base plate, it is attempted to fully utilize the compressive stress of the concrete and also to employ the torsional strength of the box-type section.
In small spans with a sufficient height, the compressive strength of the roadway slab and the base plate cannot be utilized. Dispensing with the base plate leads to a T-beam cross-section often employed in middle-sized and small spans. The roadway slab cannot be eliminated nor can its cross-section be reduced, since it provides the requisite roadway surface.
The monolithic construction of horizontal slabs and vertical or diagonal girder webs, as employed in a closed box-type cross-section and in an open T-beam cross-section, has advantages and disadvantages. With regard to small spans, the disadvangages predominate. This is particularly true when the roadway slab includes a tension region, that is, in cantilevered sections and in continuous girders in the support area. The so-called "effective slab width" to be taken into account for absorbing the bending moments is usually smaller than the overall width of the slab especially in wide bridges. The longitudinal forces developed in prestressing, however, are distributed across the entire width of the slab. As a result, in contrast to a cross-section with the smallest possible tension region, additional prestressing steel is required in the long direction and, moreover, additional steel is needed for introducing the shearing forces into the roadway slab. This is also true in certain cases if the roadway slab experiences compressive stresses.
It is true in bridges constructed in place where the roadway slabs are poured together with the webs or at least directly adjoining the webs that the advantages gained in the construction system where the cross-section is divided into successive concreting steps, cannot be utilized.